Vibratory type voltage regulators and motor speed controllers have been known heretofore. In such devices, magnetically vibratory contacts have been used to control the power output in accordance with the periodic closing of the contacts, such closing of the contacts being effective to shunt an impedance or the like from the output circuit, the closed-to-open time ratio of the contacts being adjusted by adjusting the spacing between the contacts. Such vibratory devices are operable from either A.C. or D.C. Magnetically operable choppers have also been known. One form of such chopper employes a rotating permanent magnet to oscillate a contact-carrying permanent magnet to close and open contacts. While such regulators, controllers and choppers have been useful for their intended purposes, this invention relates to a manually adjustable and controllable chopper for motor speed control.